In the past, infrared attenuating agents (IAAs) such as carbon black powdered amorphous carbon, graphite, and titanium dioxide have been used as fillers in polymeric foam boards to minimize material thermal conductivity which, in turn, will maximize insulating capability (increase R-value) for a given thickness. R-value is defined as the commercial unit used to measure the effectiveness of thermal insulation. A thermal insulator is a material, manufactured in sheets, that resists conducting heat energy. Its thermal conductance is measured, in traditional units, in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per Fahrenheit degree of temperature difference between the two sides of the material. The R value (per inch) of the insulator is defined to be 1 divided by the thermal conductance per inch. R is an abbreviation for the complex unit combination hr·ft2·° F./Btu. In SI units, an R value of 1 equals 0.17611 square meter Kelvin per watt (m2·K/W).
The heat transfer through an insulating material can occur through solid conductivity, gas conductivity, radiation, and convection. The total thermal resistance, R, (sometimes called R-value) is the measure of the resistance to heat transfer, and is determined as: R=t/k, where, t=thickness.
Rigid foamed plastic boards are extensively used as thermal insulating materials for many applications. It is highly desirable to improve the thermal conductivity without increasing the density, and/or the thickness of foam product. Particularly, the architectural community desires a foam board having a thermal resistance value of R=10, with a thickness of less than 1.8″, for cavity wall construction, to keep at least 1″ of the cavity gap clean.
It is also desirable to improve the UV stability, particularly for such as exterior wall insulation finishing system (EIFS), and highway and railway underground applications where prolonged exposure of sun light of the surface of the polymer foam boards are usually occurred in job-sites.
The physical properties of rigid polymer foam boards, such as their compressive strength, thermal conductivity, dimensional stability, water absorption rate, depend in large part on the micro-structure of the material forming the boards, i.e., the cell morphology of the foam. However, it can be difficult to control polymer foaming to the degree necessary for consistent production of a desirable cell morphology that will tend to optimize the overall foam properties, or to improve a specific property, such as the thermal insulation value of the foam
Prior art attempts to make foam micro-structures having desirable cell morphologies have included the use of nucleation agents such as powders formed from inorganic oxides, various organic materials and metals. Among these nucleation agents, the inorganic oxides, such as talc, titanium dioxide and kaolin, are the most commonly used. The size, shape, particle distribution and surface treatment of the nucleation agent(s) utilized in the process to form a foam will all tend to affect the nucleation efficiency and, consequently, the cell size morphology and distribution in the resulting foam.
Conventional methods for controlling the cell morphology, however, tend to be limited by difficulties in evenly distributing particles of the nucleation agent throughout the polymer and/or suppressing coagulation of the dispersed particles. Certain structural defects in the resulting foams are generally attributed, at least in part, to dimensional differences between the particles of the nucleating agents, which may be in the range of several microns, particularly in situations where there has been some degree of coagulation, and the desired cell microstructures, which may have a target cell wall thickness in the range of 0.2 to 6 microns, often one micron or less, for low density commercial insulation foams.
This size difference between the nucleation agent particles and the cell wall thickness may also result in relatively weak interactions between the nucleating agent and nano-scale polymer, thereby weakening the overall foam structure. Similarly, cell defects may also be attributed, at least in part, to the hydrophilic surface of most conventional inorganic nucleation agents that makes them difficult to disperse evenly in a polymer. These effects tend to result in processing difficulties, such as corrugation of the resulting foam board, when nucleation agents are added at levels greater than about 2 weight percent or the median cell size of the resulting foam is less than around 120 microns.
Prior art attempts to avoid foam structure corrugation effects have utilized cell size enlarging agents such as the waxy compositions disclosed in U.S. Pat. No. 4,229,396, the contents of which are hereby incorporated by reference in their entirety, and the non-waxy compositions disclosed in U.S. Pat. No. 5,489,407, the contents of which are hereby incorporated by reference in their entirety.
Another effort directed toward foam structures having bi-modal cell morphology (Kanelite Super EIII, Kaneka, Japan) included use of immiscible blowing agents, such as water and hydrocarbon. This combination, however, tends to result in processing difficulties due to the low solubility of water in the polymer and the reaction of water with fire retardant, such as hexabromocyclododecane (HBCD) at the elevated temperatures typically utilized during the extrusion process.
Thus, there is a need for foam products, more specifically rigid foam boards, utilizing nano-graphite particles having at least one dimension—usually the thickness of the plate shaped graphite in nano-scale, i.e., less than 0.1 microns or 100 nanometers. It is a further object of the present invention to provide a process for preparing low density extruded polymer foams and foam boards containing nano-graphite which has good processing properties, and improved foam physical properties, including thermal conductivity, ultraviolet (UV) radiation resistance, dimensional stability, mechanical strength, flame spread rate and smoke density.